This invention relates to the propagation and detection of optical signals and, more particularly, to an integrated semiconductor device that includes an optical waveguide and a photodetector.
Two basically different types of integrated waveguide/detector optical devices are known. In one type of such device, guiding and absorbing (detecting) layers are sequentially formed on top of each other on the planar surface of a substrate. The planar absorbing layer is then lithographically processed and etched to form a detector overlying a prescribed longitudinal extent of the underlying guiding layer. This type of device is described, for example, in "Waveguide-Integrated PIN Photodiode on InP" by C. Bornholdt et al, Electronics Letters, Vol. 23, No. 1, Jan. 2, 1987, pp. 2-4; in "Monolithic Integrated InGaAlAs/InP Ridge Waveguide Photodiodes for 1.55 .mu.m Operation Grown by Molecular Beam Epitaxy" by P. Cinguino et al, Applied Physics Letter 50 (21), May 25, 1987, pp. 1515-1517; and in "Monolithic Integration of An InP Inverted Rib Waveguide with a GaInAs Photodiode" by M. Erman et al, Proceedings of the 4th European Conference on Integrated Optics, 1987, pp. 28-31.
In the aforedescribed type of integrated optical device, the coupling between light propagating in the planar guiding layer and the overlying planar detector is relatively weak. Therefore, in order to obtain a high degree of light absorption in the detector, the length of the overlying detector must in practice be made relatively long. Lengths of at least about 300 micrometers (.mu.m) for such detectors are typically required. In turn, the capacitance of such a long detector is relatively high and its maximum speed of operation is consequently relatively low. In addition, high capacitance is detrimental to detector performance because it increases the noise and the minimum detectable optical signal.
In a second known type of integrated waveguide/detector optical device, an absorbing layer is grown on a substrate and then lithographically processed and etched to form a detector. Subsequently, a guiding layer is grown on the substrate in abutting relationship with respect to the detector. As a result, the coupling between the detector and light propagating in the guiding layer is very strong. A high degree of light absorption can therefore be achieved in a relatively short-length detector (for example, only about 5-to-10 .mu.m long). Such a detector exhibits a low-capacitance characteristic and is therefore suitable for use in a variety of high-speed optical signal processing applications.
The detector included in the second-described or faster device is sometimes referred to as being "edge-excited". An example of such an edge-excited detector (but not integrated on a single substrate with an associated input waveguide) is described in "High-Speed Zero-Bias Waveguide Photodetectors" by J. E. Bowers et al. Electronics Letters, Vol. 22, No. 17, Aug. 14, 1986, pp. 905-906. An example of an integrated waveguide plus photodetector in the "edge-excited" configuration, showing the need for two separate epitaxial growths, is described in "Integrated Waveguide p-i-n Photodetector by MOVPE Regrowth," by S. Chandrasckhar et. al., IEEE Electron. Dev. Lett. Vol. EDL-8, No. 11, Nov. 1987, pp. 512-514.
Improved performance of integrated optical devices of the two types described above is usually obtained if the aforementioned guiding and absorbing layers are formed by standard epitaxial growth processes. The guiding and absorbing layers of the first-described or slower type of device can both be formed in sequence in a chamber in a single epitaxial growth cycle. This considerably simplifies the fabrication of the device. By contrast, the epitaxial guiding and absorbing layers of the second-described or faster device must be formed in two distinct epitaxial growth cycles separated by lithographic processing and etching of the absorbing layer. This considerably complicates the fabrication of the faster or edge-excited integrated device.
Accordingly, efforts have been directed by workers skilled in the art aimed at trying to devise an integrated waveguide/detector device characterized by both high speed and ease of fabrication. It was recognized that these efforts, if successful, would contribute significantly to the realization of high-performance low-cost optical signal processing systems.